Separation of very fine liquid droplets from a gas is required in many applications where finely dispersed liquid droplets are used in chemical or energy processes.
One example involves the fine pulverization of water prior to admission into the suction side of a compressor, aiming at increasing the effectiveness of a (turbo) compressor by cooling the gas before admission. Even if it is assumed that cooling-evaporation consumes 70-80% of the liquid dispersed into atomized droplets, approximately 20% of the liquid in the form of fine droplets remain and enter the combustion chamber (of the gas turbine power equipment). Due to the resulting “humid” nature of the combustion gas, the system efficiency, while considerably increased by cooling the air prior to compression, may be reduced by 2% or more. In the case of a 10 MW gas turbine unit such a reduction in efficiency represents a significant amount of energy (which is consumed as latent heat for evaporation in the combustion chamber).
In a second example, a fine pulverization is required to increase the contact area of a liquid reactant in order to improve the contact area in a chemical reaction (e.g., 1 liter=1 dm3 of liquid pulverized to a 5 μm droplet size will acquire an exchange area of approximately 4800 m2).
In a third example, aiming at the removal of extremely fine solid particles, a particle cloud is “chased” by pulverized liquid droplets which are formed by a pulverization process. A correlation is required between the size distribution of the solid particles and the size distribution of the liquid droplets (which “chase” and coalesce with the “dust-like” solid particles) in the range of “a similar order of magnitude” (e.g. micron for micron).
In a fourth example a gas may be selectively separated using a non-contact (surface) gas extraction device. Atomization of a fine cloud of a selective adsorbent/absorbent in the form of a dense cloud of micro-droplets will represent a solution which will avoid the need for film or solid support surfaces.
In a fifth example, liquid micro-droplets may result from a process of bulk condensation, where a humid gas (containing water or any other solvent in a gaseous form) is exposed to a pressure-temperature process and with the aid of a large population of sub-“micronic”impurities (usually present in any industrial gas) offers conditions for “bulk condensation” of the liquid micro-droplets.
In a sixth example, a number of technologies can be grouped together in the field of “direct contact heat & mass exchangers” which may be used to avoid the use of conventional bulky equipment, fouling, corrosion, large capital & operation costs and to take full advantage of existing or created micro-droplets of liquid for contacting gas or solids (in fine particulate form) and/or for the enhancement of chemical reactions, evaporation processes, heat transfer processes and mass transfer processes.
Although the technology of atomizing or pulverizing liquids into droplets is well represented in the technical literature (see for example “Atomization & Sprays”, A. H. Lefebre, printed by Taylor & Francis-Hemisphere, 1989), the next important stage of almost any such modern processing system, consisting of the effective separation of the “processed”or created micro-droplet population (usually suspended by a gas), is not well developed, is difficult and represents the main deterrent for a broader application of direct-contact technologies (micro-droplets of liquid direct contacting gas and/or dust-like micro-particles).
In any separation technology a proper balance between separation efficiency, maintenance cost and minimization of pressure drop, whether in a clean or clogged state, is essential. Some exemplary separation technologies disclosed in the art include the following:                (a) particles to be separated are electrically charged prior to entering the separator apparatus and meet walls carrying an opposite electrical charge (electrostatic or AC/DC). Aqueous droplets are generally avoided due to high-electrical conductance and other electrically related safety concerns, with the result that electrically based separation technologies typically cannot be used for separating aqueous or highly electrically conductive liquids. These technologies do, however, provide a “wall extraction” but in a laminar, quiescent flow regime, which detracts from the system efficiency but enhances the particle removal mechanism;        (b) filters and coalescers, metallic and non-metallic pads and micro-porous filled containers for liquid droplets and solid particles may represent viable alternatives for some applications. These technologies can be used in tailored applications but require frequent maintenance, particularly where impurities are attached to any of the phases of the fluid system being treated. Clogging is one of the more important problems associated with these types of separation technologies. Where “plugging” impurities are attached to one or more phases of the fluid system being treated, and where large amounts of gas throughput should be processed with minimum pressure drop, the use of micro-porous container or pads system is typically excluded, thus eliminating the application of these types of technologies from fluid systems carrying “gum-like” suspensions (as in oil/gas fields), which have the tendency to rapidly deteriorate the flow-pressure drop characteristics of the fluid flow and render the technology inefficient or unacceptable;        (c) mechanical separation technologies may be used to separate liquid droplets from some fluid systems, but the separation of liquid micro-droplets entrained by a gas is known to pose practical problems with most conventional mechanical separator designs including gravitational separators which depend upon gravity settling and according to Stokes' Law require a residence time (Liquid Volume (m3)/Throughput in (m3/h)) in excess of the time required for a liquid particle to reach the liquid-gas interface. For example, for a liquid micro-droplet having a size of 5 μm (1 μm=1 m/106), a free falling velocity in air is obtained (from Stokes' Law) according to Equation 1:        
                              U          droplet                =                                                                              d                  p                  2                                ⁡                                  (                                                            ρ                      liq                                        -                                          ρ                      air                                                        )                                                            18                ⁢                η                                      ⁢            g                    =                                                                                                                                        (                                                                              5                            /                                                          10                              6                                                                                ⁢                          m                                                )                                            2                                        ⁢                                          (                                              1000                        -                        1                                            )                                        ⁢                                          (                                              k                        ⁢                                                                                                  ⁢                        g                        ⁢                                                  /                                                ⁢                                                  m                          3                                                                    )                                                                                                  18                      ⁡                                              [                                                                              (                                                          0.02                              ⁢                              cP                                                        )                                                    /                          1000                                                ]                                                              ⁢                                          (                                              kg                        ⁢                                                  /                                                ⁢                        ms                                            )                                                                      ⁢                9.81                ⁢                                  (                                      m                    ⁢                                          /                                        ⁢                                          s                      2                                                        )                                            ≈                              0.06                ⁢                                  /                                ⁢                100                ⁢                                  (                                      m                    ⁢                                          /                                        ⁢                    s                                    )                                                      =                          0.6              ⁢              mm              ⁢                              /                            ⁢              s                                                          (        1        )                             where Udroplet is the free falling velocity, dp is the spherical diameter of droplet/particle (falling under Stokes' Law), μ is the viscosity in SI units (1 cP=1/1000 kg/m s), and ρ is the densities of water (for the water droplets) and gas.        
For a gas space of 0.5 m, a 5 μm droplet will require approximately 1000 s (16 minutes) to reach the liquid level, for a 2 μm micro-droplet, the required time (in absolutely still air) is more than 30 min.
Conventional (gravity/cyclone) separation are customarily designed for a “free gas” velocity of approximately 0.1-0.3 m/s. At this order of velocity magnitude, all droplets having a free falling velocity an order of magnitude smaller will typically be entrained and will not fall and separate. Therefore, any technology using a “gravity separation mechanism” is not typically feasible for the separation of liquid micro-droplets from gas streams.                (d) cyclone, rotational, and other inertial separation technologies may also be used to separate liquid droplets in some applications. In these technologies, the effect of separation may be intensified using a “cyclone” or other inertial effect. This may be visualized if, in Equation (1) the acceleration due to gravity (g=9.81 m/s2) is replaced by centrifugal acceleration Rω2 (m/s2). Measured as a “multiple of “g”, centrifugal acceleration is practically limited to about 5-10 times “g” (or a maximum of 40 g for extremely expensive separation units and about 100 g for special “multiple plate designs”). Even if a “10 g” separation apparatus is utilized, the centrifugal acceleration achieved may not be high enough for effective separation of micro-particles.        
A self-generated (swirl flow) cyclone will typically achieve relatively low “g” values unless extremely high pressure drops are acceptable in the system.
Another solution would be to create a “compact” unit where the “free falling distance to interface” is significantly reduced (to be in the order of about 1 centimeter) in order to reduce the required residence time for separation. This approach is used for some special (heavy oil) liquid-liquid-solid separators, where the viscosity of the continuous phase (i.e., the carrier) is a deterrent to the use of other technologies.